The art of lithographic printing is based on the immiscibility of ink, generally an oily formulation, and water, wherein in the traditional method the ink is preferentially retained by the image or pattern area and the water or fountain solution is preferentially retained by the non-image or non-pattern area. When a suitably prepared surface is moistened with water and an ink is then applied, the background or non-image area retains the water whilst the image area accepts ink and repels the water. The ink on the image area is then transferred to the surface of a material upon which the image is to be reproduced, such as paper, cloth and the like. Commonly the ink is transferred to an intermediate material called the blanket which in turn transfers the ink to the surface of the material upon which the image is to be reproduced.
New types of "waterless" lithographic printing employ only an oily ink material and preferentially ink-accepting image areas and ink-repelling non-image areas on the printing form.
A generally used type of lithographic printing form precursor (by which we mean a coated printing form prior to exposure and development) has a light sensitive coating applied to an aluminum base support. Negative working lithographic printing form precursors have a radiation sensitive coating which when imagewise exposed to radiation of a suitable wavelength hardens in the exposed areas. On development the non-exposed areas of the coated composition are removed leaving the image. On the other hand positive working lithographic printing form precursors have a radiation sensitive coating, which after imagewise exposure to radiation of a suitable wavelength becomes more soluble in the exposed areas than in the non-exposed areas, in a developer. In both cases only the image area on the printing form itself is ink-receptive.
The differentiation between image and non-image areas is made in the exposure process where a film is applied to the printing form precursor with a vacuum to ensure good contact. The printing form precursor is then exposed to a light source, comprising UV radiation. In the case where a positive form precursor is used, the area of the film that corresponds to the image in the printing form precursor is opaque so that no light will strike the printing form precursor, whereas the area on the film that corresponds to the non-image area is clear and permits the transmission of light to the coating which becomes more soluble and is removed.
The resists used in pattern forming methods for electronic parts such as printed circuits are also classified into two types: negative working and positive working. After exposure to radiation and development, the resist pattern is used as a mask for forming the patterns onto the underlying electronic elements - for example by etching an underlying copper foil. Due to the high resolution demands and the requirements of high resistance to etching techniques, positive working systems are widely used. In particular, in the main there have been used alkali developable positive working photoresists mainly composed of alkali-soluble novolac resins as disclosed in J.C. Streiter, Kodak Microelectronics Seminar Proceedings, 1979, p. 116. The primary active component of such positive working compositions, both in the context of lithographic printing forms and electronic parts, is a naphthoquinonediazide (NQD) derivative.
The types of electronic parts whose manufacture may use a photo resist include printed wiring boards (PWBs), thick- and thin-film circuits, comprising passive elements such as resistors, capacitors and inductors; multichip devices (MDCs); and integrated circuits (ICs). These are all classified as printed circuits.
A mask may be used in imaging methods, the required resist pattern being formed on the mask, which is then used as a screen in a later processing step.
Neuman, U.S. Pat. No. 4,708,925 (3M) describes a radiation sensitive element, e.g., a printing plate, having coated on it a composition stated to be photosolubilisable, comprising an alkali-soluble phenolic resin and an onium salt. The onium salt imparts developer resistance to the phenolic resin, this developer resistance being removed upon exposure to radiation, and consequent photolytic decomposition of the onium salt. Suitable onium salts are said to include iodonium, sulphonium, bromonium, chloronium, oxysulphonium, sulphoxonium, selenonium, telluronium, phosphonium and arsonium salt. Of these, iodonium, sulphonium and oxysulphonium are said to be preferred, especially iodonium. The maximum sensitivity of the composition is generally in the ultraviolet region but it is stated that the sensitivity can be adjusted by addition of one or more spectral sensitizers, of which many examples are listed.
EP-A-625 728 (Kodak) describes a radiation sensitive composition suitable for a printing plate, sensitive to both ultraviolet and infra-red radiation and capable of functioning in either a positive working or negative working manner, comprising (1) a resole resin, (2) a novolac resin, (3) a latent Bronsted acid and (4) an infra-red absorber. To utilize the printing plate as a positive-working plate requires that it be imagewise exposed to activating radiation, thereby rendering the exposed areas alkali-soluble. To utilize it as a negative working plate requires the steps of imagewise exposing to activating radiation, and heating the plate to provide reduced solubility in the unexposed regions. In achieving these properties the use of a mixture of a resole resin and a novolac resin is said to be essential.
Suitable latent Bronsted acids for use in the invention of EP-A-625 728 are said to include onium salts, in particular iodonium, sulfonium, phosphonium, selenonium, diazonium and arsonium salts. Use of diazonium salts is said to be preferred due to their equivalent sensitivity to other latent Bronsted acids in the infra-red region and higher sensitivity in the ultraviolet region.